The use of plastic containers as a replacement for glass or metal containers in the packaging of beverages has become increasingly popular. Several types of plastics have been used, ranging from aliphatic and aromatic polyolefins (polyethylene, polypropylene, polystyrene) over halogenated polymers (polyvinyl chloride, polyvinylidene chloride) and aliphatic polyamides (nylons) to aromatic polyesters. As far as the rigid food packaging sector is concerned, polyethylene terephthalate (PET), an aromatic polyester, is by far the most widely used resin. This choice is driven by its unique material properties, combining amongst others shatter resistance, lightweight, high mechanical strength, transparency, recyclability, . . . . Beverage applications, both for carbonated and non-carbonated products, constitute the single largest application area for PET containers. Most PET containers are made by stretch blow molding of preforms which have been made by processes including injection molding. In some circumstances, it is preferred that the preform resin is amorphous or only slightly semi-crystalline in nature, as this allows for stretch blow molding. Highly crystalline preforms generally are difficult, if not impossible to stretch blow mold.
With plastic materials (like PET) being derived from oil, the ongoing increases in resin, oil and energy pricing has created significant pressure on package owners to reduce the total cost of ownership of their plastic packaging mix. This in turn drives focus on finding solutions which enable to further reduce the wall thickness of these plastic (like PET) containers (light-weighting) whilst maintaining the inherent overall performance characteristics and design flexibility. It also challenges the plastic material converting industry to increase the output of its plastic material converting platforms, on processes like injection and stretch blow molding. The combination of reduced material utilization and increased production manufacturing output reduces the total cost of ownership for both preforms and containers.
At the same time, in some specific end market applications, increased performance specifications are requested on parameters including thermal stability, barrier performances and mechanical rigidity. Such a specific end market application requesting increased performance specifications for PET containers includes hot-fill containers, which must withstand filling with hot liquid products without significant deformation, followed by sealing and a cooling process which creates a vacuum in the container, due to the volume contraction of the hot filled liquid.
A particular problem associated with these hot-fill containers concerns the thermal stability of both the body, but especially the neck finish of the container throughout the hot filling process, because increase in temperature during the process induces molecular relaxation and shrinkage in the container material. The higher the crystallinity of the container, the more the container is resistant to said relaxation. When an essentially amorphous or only slightly semi-crystalline preform is converted into a container by the stretch blow molding process, the process conditions determine the amount of crystallinity that is induced in the different container parts. Unless special precautions are taken and/or additional process steps are included, the neck finish, being clamped and restricted from stretching, will receive almost no increase in crystallinity. Any increase obtained will always be negligible in comparison to the increase induced in the stretched main body. Any container part made entirely of amorphous or only slightly semi-crystalline PET may not have enough dimensional stability during a standard hot-fill process to resist the relaxation process and hence meet the specifications required when using standard threaded closures.
Unacceptable volume shrinkage of the container and/or especially of the neck area may create leaks between the neck and closure, thus increasing exposure to micro-organisms, whilst increasing gas ingress and/or egress. This can lead to non-specification compliant quality issues and, in case of food applications, to potentially consumer hazardous situations when pathological micro-organisms are able to grow inside the packed food matrix.
In these circumstances, a container comprising increased amounts of crystalline PET, especially in the neck finish, would be preferred, as it would hold its shape during hot-fill processes.
Another application in which plastic containers are subjected to elevated temperatures, include pasteurisable containers which, after filling and sealing, are then exposed to an elevated temperature profile for a defined time period. Throughout the pasteurization process, the sealed container must have dimensional stability so as to remain tight and within the specified volume tolerance.
Yet another high-temperature application is the use of plastic returnable and refillable containers for both carbonated and non-carbonated beverages, whereby the container must withstand wash and reuse cycles. Such containers are filled with a carbonated or non-carbonated beverage, sold to the consumer, returned empty, and washed in a hot, potentially caustic solution prior to refilling. These repeated cycles of thermal exposure make it difficult to maintain the overall shape, appearance and threaded neck finish within the tolerances required to ensure adequate functionality and/or general consumer acceptance.
A number of methods have been proposed to address said problems of elevated temperature impact on plastic containers throughout their filling or use cycle, thereby ensuring that the required specifications for volume shrinkage, shape retention, neck softening and others are met.
One such method consists of adding an additional manufacturing step that exposes the neck finish and/or body part of the preform or container to a heating element in order to thermally crystallize the neck finish and/or body part of the preform or container. However, the required capital investments, the increased manufacturing processing time and costs for specific materials and/or auxiliaries lead to an increased overall cost of ownership and increased total product cost. As previously stated, the overall cost of producing a container is very important and needs to be tightly controlled because of competitive market and business pressures.
Alternative methods of strengthening the neck finish involve crystallizing select portions of the neck finish, such as the top sealing surface and flange. Again, this requires an additional heating step and increased processing time.
Another alternative is to use a high glass transition temperature material in one or more layers of the neck finish. Generally, this involves more complex preform injection molding procedures to achieve the necessary layered structure in the finish.
Another alternative method includes specific container design and design features such as to compensate for the developed vacuum through the hot-fill process.
A particular performance characteristic associated and critical to carbonated beverage containers, include barrier performance i.e. the control of gas ingress and/or egress. To conserve the taste of the beverage and hence increase the shelf life of the product, it is essential that the gas mixture in the container remains unchanged for as long as possible after the filling process. Different methods are being used nowadays to enhance the barrier properties of the container walls, including passive methods (co-extrusion multilayer approaches, coating applications, nanotechnology) and active methods (oxygen scavenger incorporation) and combinations thereof. All these methods significantly increase the cost of ownership.
With respect to mechanical properties, commercial articles in general made out of polyesters and more specifically out of PET depend primarily on some degree of orientation induced during the manufacturing processes to enhance the mechanical properties.
The degree of molecular orientation and the physical properties of the resulting oriented article are governed o.a. by the strain rate applied during processing, by the stretch ratio, by the molecular weight of the resin and by the temperature at which the orientation takes place. Bi-axial orientation during stretch blow molding when transforming a preform into a container leads to strain induced crystallization. This in turn improves mechanical strength and barrier properties. The amount of crystallinity reached and the crystal shape depend on the strain rate and the stretching temperature. State of the art production methods are optimized to enhance the mechanical strength by stretching the amorphous preform to maximal strength within the limits of the material characteristics. Typical average applied stretch ratios amount to up to 4.5 in the circumferential direction and up to 3.2 in the axial direction. Exceeding these limits and entering ranges of too high stretch ratios lead to the creation of micro voids and premature container failure.
A particular problem when blow molding remains generating enhanced mechanical strength in the neck finish and in the bottom portion of the container in light of the negligible respectively low stretch ratios in these specific areas.
Especially in the case of containers intended for filling with carbonated soft drinks this local reduction in strength leads to more severe container deformation and consequently to a reduction of dissolved carbon dioxide in the soft drink and to a decreased shelf life. To alleviate the inherent weakness of these particular areas recourse is taken to preforms exhibiting significant higher wall thicknesses in neck finish and bottom area.
Another widely used method to capitalize on the induced crystallinity and to extend it into less oriented areas is the process called heat-setting, in which the transformation from amorphous preform to crystalline container is preformed at high temperature for rather prolonged exposure cycle times.
A particular limitation, state of the art production methods suffer from, stems from the preheating prior to stretch blow molding the container and more specifically to the heat history heat-set containers are subjected to.
In the heat-set process the preform and resulting container are exposed to significantly higher temperatures then is the case for so-called cold-drawn bottles, as e.g. used for water and carbonated soft drinks. A typical preform reheat temperature for a heat-set container amounts to 130° C. versus 90-100° C. for cold-drawn containers.
Following the preform is stretch blown in a heated container blow mold where only the inner container wall is air cooled.
Typical heat-set container mold temperatures are in the range of 160° C. In contrast a cold-drawn container is blown in a mold kept at around 20°.
This thermal treatment destroys most of the stretch induced orientation as relaxation processes have ample time to develop. As a consequence the resulting heat-set container looses a substantial amount of mechanical strength. The ultimate mechanical strength reached in the heat-set bottle is achieved predominately by additional crystallization through the prolonged thermal treatment.
Overall the resulting heat-set container strength is lower than that of a typical cold-drawn container.
Therefore heat-set containers necessitate higher material demand, longer process cycle times and the application of more energy compared to cold-drawn containers.
From the above it is clear that it would be desirable to provide a method of manufacturing a preform made out of crystallizable polymers for a container having a neck finish which resists deformation, particularly at elevated temperatures, characterized in that it is produced within the standard processing time frame and/or limited extensions thereof.
It is equally clear that it would be desirable to provide a method of manufacturing a preform made out of crystallizable polymers for a container having, at optimized wall thicknesses, equal or superior end performance properties including, amongst others, gas permeation resistance and mechanical strength.
According to a first embodiment of the present invention, the present invention is directed to a method for making out of crystallizable polymers an article in general, or more specifically a preform and resulting stretch blow molded container, providing equal or superior end performance characteristics. Said method includes hot runner system modifications, thereby inducing new structures both at the level of preform and/or container.
Another embodiment of the present invention provides a method and apparatus for the cost-effective manufacture of such articles in general, specifically injection molded preforms and stretch blow molded containers.